1. Field of the Invention
This invention generally relates to systems configured to inspect a wafer.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail.
In the cases of shot noise limited operation as well as wafer noise limited operation, the achievable signal-to-noise ratio of a surface inspection system, at constant throughput, is known to scale as the square root of the number of illuminating spots on the wafer. For example, when a system with one spot scans a wafer and achieves a signal-to-noise ratio on a particular defect of S, a corresponding system that scans a water with two spots, each of which is half the area of the original spot, can achieve a signal-to-noise ratio of sqrt(2)*S, assuming all other system parameters remain constant.
This advantage argues for enhancement of sensitivity by increasing the number of illuminating spots substantially, by using spatial multiplexing (multiple spots at the same instant in time). Such a technique has been disclosed in patents such as U.S. Pat. No. 7,385,688 issued on Jun. 10, 2008 to Kadkly et al., which is incorporated by reference as if fully set forth herein.
In the case of spatial spot multiplexing or spatial domain multi-spot, the signals from the spots are all generated in time simultaneously or nearly simultaneously. Therefore, in order to separate these optical scattering signals, typically the collecting optics is constructed to separately resolve the scattering from each spot and to direct and/or image each spot onto a separate optical detector such as a photomultiplier tube, photodiode, charge coupled device (CCD), avalanche photodiode (APD), CMOS sensor, etc. For instance, with three incident spots, three separate detectors could be present, with corresponding sets of optics including, but not limited to, lenses, attenuating filters, wavelength selective filters, spatial apertures, and polarizing elements.
Accordingly, it would be advantageous to develop improved systems for multi-spot wafer inspection.